Tomato, one of the major vegetables, is widely cultivated in the world. In 2004, the production of tomato worldwide reached 120 million metric tons. The United States ranks second in tomato production with 428,900 acre of planted area, and total value was over 2 billion dollars in 2006. In the US, Ohio ranks third for both processing (6,400 acres) and fresh tomato production (6,700 acres) with total value of 125 million dollars in 2006. Clavibacter michiganensis subsp. michiganensis (Cmm), the bacterial pathogen of tomato canker, causes severe economic losses in commercial tomato production in the US and worldwide. Loss of yield is mainly due to defoliation, wilting and death of plants, reduced fruit size, and lesions on fruit. The disease was first discovered in a Michigan greenhouse in 1909 and many occurrences have been reported since then in North America. Bacterial canker is considered by the greenhouse tomato industry, which produces about 35% of all tomatoes sold in supermarkets and similar venues in North America, to be the most important and costly disease it must manage. Cmm is a quarantined pathogen in Europe and infected greenhouse and field plantings must be destroyed. Severe epidemics may cause 80% yield loss and according to the 2005 EPPO report tomato canker is present almost in all tomato production areas around the world. Bacterial canker is very difficult to control once it has been established in a planting, and to date there are no antibiotics or bactericides available to kill Cmm in the plant and prevent its spread to other plants.
An important means of disease spread is through infected seed and seedlings. Even a low transmission rate (0.01%) from seed to seedling can cause an epidemic of the disease under favorable conditions. Tomato seedlings are usually kept in a nursery for 4-6 weeks before transplanting to either greenhouse or field. As the density of seedlings is very high, the bacteria are easily spread through irrigation and leaf-to-leaf contact. Some infected young plants die soon after transplanting but many survive with epiphytic populations of Cmm that may spread to other plants. The mechanism of seed to seedling transmission of Cmm and the significance of its epiphytic phase in the ecology and epidemiology of the disease are still not fully understood.